Security paper with color mark

ABSTRACT

A security paper is provided for incorporation in a security document, or other document having intrinsic value. The security paper includes a resinous substrate sheet on which indicia are printed. Paper sheets are laminated on either side of the resinous substrate sheet using a suitable adhesive. In the laminated security paper, the indicia printed on the substrate sheet are undetectable when viewed in reflected light, but become apparent when viewed transmitted light within the visible spectrum. The security paper may be incorporated in a security document in which a set of indicia printed on at least one of the outer faces of the paper sheets, such that the indicia on the substrate and the indicia on the document form a total image when viewed in transmitted light. A method for manufacturing the security paper is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to security papers and, moreparticularly, to a laminated security paper for use in making banknotes,passports, stock certificates, checks and the like.

2. Description of Related Art

A great number of printed documents require highly reliable means ofensuring their authenticity due to some intrinsic value associated withthem. These documents range from banknotes to property titles, andinclude such things as negotiable instruments, stock certificates,checks, and passports. To be truly of value to the user or holder of thedocument, the means employed to indicate authenticity must be readilyand easily detectible. Moreover, to serve the more general purpose ofallowing the public at large to rely on the authenticity of thedocuments, these means should be permanent, durable, and difficult toreplicate. This latter quality is particularly important to preclude, orat least to dissuade, attempts at counterfeiting the documents in orderto ensure a maximum degree of confidence in the original document. Inthe case of banknotes, passports,, checks, and other intrinsicallyvaluable documents, confidence in the authenticity of the document isespecially important, as any member of the public might become a holderor user of the document at any time. Thus, creators of suchintrinsically valuable documents are most often concerned with providinga means by which the public may verify the authenticity of the documentsbecause, ultimately, it is the unsuspecting public who is most at riskof being duped by counterfeits.

Historically, attempts at guaranteeing the authenticity of intrinsicallyvaluable documents have concentrated on two aspects of the documents:(1) the paper employed as the basis for the document, typically referredto in the art as "security paper," and (2) some security feature eitherincorporated in the paper or added during the transformation of thepaper into the final document. Due to their important role in providinglasting documents and a dependable basis for public confidence in theauthenticity of such documents, both the security paper and the securityfeature must respond to numerous, stringent criteria relating to theirdesign and utility. Criteria for security papers include, both physicaland optical requirements. The requirements of security features relatenot only to their physical properties, but also to the relativedifficulty of their replication and to their reliability in guaranteeingthe authenticity of the final document.

The physical requirements imposed on security papers, particularlypapers used in the manufacture of banknotes, are generally much morestrict than for other types of paper. One such requirement is the "basisweight" or the "grammage" of the paper, usually expressed in grams persquare meter, or pounds per ream. For most security papers, the basisweight must fall within a fairly narrow range so as to lend sufficientbody, or feel, to the document to satisfy the expectations of holders,while allowing the document to be easily folded and unfolded. Closelyrelated to the basis weight is the "caliper," or thickness of the paper,expressed in fractions of a millimeter or inch. It is important that thecaliper of the security papers used in any particular applicationfunction well with machines, such as automatic teller machines and highspeed sorters, which will inevitably handle the final documents.

Strength and durability are crucial properties of security papersbecause they give the final document its ability to withstand the use,and abuse, likely during its service life. The tensile strength,expressed in units of force per unit width, represents the greatestlongitudinal stress a piece of paper can bear without tearing apart.Other strength parameters include wet tensile strength, breakingstrength, tearing strength, and folding endurance. Because banknotes areinevitably folded and unfolded many times during their circulation life,high strength and great folding endurance are required of the papersfrom which they are made. Banknote paper is typically designed to allowsome 5,000 to 8,000 folds during its expected circulation life. Itshould be noted that ratings of security papers based upon the number offolds the papers can endure without tearing may vary widely dependingupon the test procedures employed. Moreover, while permitting aconsiderable number of folds, modern banknote paper must retain somedegree of rigidity, or resistance to bending, sometimes referred to as"crispness," to pass successfully through automatic teller machines andhigh speed sorters.

Other important physical properties of security papers include porosity,printability, and dimensional stability. Because porosity is related tothe soilability of a paper, that is, the tendency to absorb or retainimpurities, a relatively low porosity is generally desirable forsecurity papers, to increase the likelihood that the document willmaintain a clean appearance throughout its useful life. At the sametime, security papers require good printability characteristics toensure accurate and precise impression in the printing processesemployed in the manufacture of the final document, as well as properpenetration and adhesion of inks used in the document production.Printing processes used to make certain intrinsically valuable documentsmay place extreme demands on the paper. Intaglio, or gravure printing,for example, often used in printing banknotes and other documents,involves tremendous pressures between the printing plates and the paper.The paper selected must withstand these processes without loss ofphysical integrity. Finally, the paper's dimensional stability isimportant, both during the manufacturing processes and during theservice life of the final document. In particular, the paper should notswell or contract excessively due to changes in humidity that typicallyoccur during the manufacturing processes and during ordinary use.

Security papers typically exhibit certain optical properties that affecttheir usefulness and acceptability to the public. The most important ofthese properties are the surface characteristics of the paper, and thepaper's opacity. Surface characteristics include color, brightness,gloss, finish and smoothness. These parameters are usually specified bythe designer of the final document based upon the aesthetics desired inthe document; and various processes, such as coating and calendering,may be used to obtain the desired appearance and feel. The opacity ofthe paper refers to the relative inability of light to pass through thepaper. Opacity is generally represented in terms of the percentage ofthe light incident on one side of the paper that is not transmittedthrough the paper to an observer on the other side, but is reflected orabsorbed by the paper. High opacity, that is, in excess of about 75%, isgenerally desirable for security papers. However, variations in opacitymay actually serve as security features in the final document, as in thecase of watermarks, wherein the image or mark, visible in transmittedlight, is formed by causing the paper to take on different levels ofopacity in such a way as to define a recognizable mark.

The selection and specification of the security feature to beincorporated into an intrinsically valuable document is often intimatelyrelated to the specification of the security paper in connection withwhich it is to be used. There are several reasons for this. First, thesecurity feature is often a quality of the paper itself, such as awatermark or a particular paper composition. Even where the feature maybe said to constitute an element separate from the paper, such featuresare often imbedded in the paper during the papermaking process. This istrue for a great number of security features, including so calledsecurity threads, chemical additives, colored confetti, and thin tapescontaining microtext or other indicia. In addition, even surface-appliedsecurity features must be compatible with the underlying paper toprovide the desired adhesion and durability characteristics to theresulting document. Such surface-applied security features includenon-reproducible images, such as line art that produces moire patternswhen photocopied, photosensitive coatings, and surface decals. Evenintricate line art printed on the face of the document may constitute asecurity feature by virtue of the difficulty with which it isreplicated. In all cases, however, the security feature must functionwell with the security paper selected.

The criteria for an effective document security feature are relativelyeasy to formulate. Such features should be difficult to replicate todeter potential counterfeiters, or at least to render the fruit of theirenterprise less passable to the public. The features should permit readydetection by means available to ordinary holders or users of the finaldocument. For banknotes and other documents on whose authenticity thepublic at large relies, the features should be discernible andverifiable under ordinary light conditions. Finally, in certain cases,it may be desirable that the security feature afford detection of themovement of large quantities of documents, such as banknotes.

While these criteria may appear straightforward, developing a securityfeature that reliably satisfies them is not an easy task. Historically,the production of an effective security feature for documents reliedupon specialized knowledge or ability possessed by only a tiny segmentof the public. Thus, for centuries the rare talents of the masterengraver and the specialized knowledge of the printer sufficed toprevent or deter most forgery attempts. Later, the combination ofspecialized knowledge of several disciplines into the same documentbecame the key to preventing counterfeiting by all but the mostdetermined and able teams of forgers. However, modern advances in colorphotocopying, scanning, and offset printing technologies have made theproduction of very convincing counterfeits possible even without suchspecialized knowledge. To be truly effective against forgery, securityfeatures must now be not only difficult to replicate due to thespecialized knowledge required for their production, or due to acombination of specialized fields into a single document, but must bevery difficult or impossible to duplicate on sophisticated photocopyingmachines.

Finally, to be useful in producing intrinsically valuable, documents,the security paper and the security feature incorporated in or on thepaper must together be capable of transformation into a aestheticallypleasing document, at commercial rates and at a reasonable cost. Thegovernmental, quasi-governmental, and corporate entities that generallyissue such documents demand no less than that their documents reflectthe character of their issuer and inspire the utmost confidence by boththeir functionality and their appearance. Thus effective security papersand security features should offer a wide degree of freedom in thedesign and execution of the final document.

A number of approaches have been proposed in the past for providing asecurity paper with effective security features. May of the proposedsecurity papers incorporate security features during the papermakingprocess, that is, on the papermaking machine while the web of thesecurity paper is being formed. Such papers make use of securityfeatures such as water marks, security threads, security tapes with orwithout microtext, confetti, and other devices embedded in the paperprior to completion of the papermaking operation. However, such securitypapers generally cannot provide a security feature covering the entiresurface of the security paper due to the need to drain moisture, fromthe paper during final processing. In addition, security features suchas confetti are not generally localized within the security paper, as isrequired in many applications. Although security features such assecurity tapes and threads may be localized in the papers, these devicescan cause difficulties in printing, sheeting and other process steps dueto the increased local thickness in the paper resulting from theirpresence.

Another approach to incorporating a security feature in security paperis to laminate finished paper sheets to form a composite security paper.One such security paper is described in U.S. Pat. No. 5,161,829. In thatsecurity paper, at least two paper sheets are laminated to form asecurity paper with an authentication indicator incorporated in thelaminate. The authentication indicator is printed on the inner surfaceof at least one of the paper sheets prior to lamination, and isdetectible in transmitted light, but not in reflected light. However,printing authentication indicators on one of the paper sheets can leadto problems with "show-through," or bleeding of the inks used to printthe authentication indicators, through the paper sheets to the surfaceof the security paper. Furthermore, where printing is done on a thininner paper sheet, this would usually involve the use of thin inks,resulting in substantial masking of the security feature by surfacepriming and making the security feature less visible or not visible atall. Moreover, the use of paper sheets in the laminate does not resultin a composite security paper having physical properties beyond thoseattainable with more conventional single-web papers. In particular,properties related to circulation life, including folding endurance andtearing strength, are in no way enhanced in the resulting laminate.

Another laminated paper product incorporating a security feature isdescribed in Japanese laid-open application no. 322109/88. That product,a card stock formed by laminating two relatively thick paper sheets oneither side of a synthetic substrate, incorporates machine-readablemarks on the substrate material. The marks, such as universal productcodes, are printed in inks that do not permit transmission of light ofnear infrared wavelengths. Other marks are printed on the substrate innear infrared-transmitting inks. The resulting marks are notdistinguishable in visible light, but can be distinguished and read whenthe cards are passed between a light emitting diode of the properwavelength and a photoreceptor. However, the cards described are notintended, nor are they suitable for use as security paper, due to thestiffness and low folding endurance inevitably resulting from theirphysical structure. Moreover, the security feature taught in thisdocument is not readily discernible to a holder under ordinary lightconditions. On the contrary, the marks printed on the substrate cannotbe discriminated in visible light and are thus of no use to the ordinaryholder without special detection means.

The present invention is directed to overcoming or minimizing thedrawbacks of the existing techniques set forth above, and to providing asecurity paper for documents with intrinsic value having securityfeatures that can be readily distinguished by the public, but that arenot susceptible to replication by counterfeiters. The invention providesa security feature in the form of a color mark, not obtainable by priorart processes or in known security papers. Furthermore, the inventionoffers a security paper of substantially uniform thickness, facilitatingconversion operations such as printing and sheeting, and allowing thepaper to perform well in high speed sorters and automatic tellermachines. Moreover, the invention provides security features which maybe localized where desired. The security feature may be employed aloneor in conjunction with known security devices.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided asecurity paper that includes a resinous substrate sheet having twofaces. Indicia are disposed on one of the faces of the resinoussubstrate sheet, and two paper sheets are permanently laminated on eachof the faces of the substrate sheet. In the resulting laminate, theindicia on the substrate sheet are substantially undetectable whenviewed in reflected light and are apparent through the security paperwhen viewed in transmitted light within the visible spectrum.

In accordance with another aspect of the present invention, there isprovided a laminated security paper having a resinous substrate layerwith a first and a second face. A layer of color indicia is disposed onthe first face of the substrate layer. The layer of color indiciaincludes at least one region having a color corresponding to awavelength of light from 380 to 720 nanometers. A first adhesive layeris disposed on the first face of the substrate layer over the layer ofcolor indicia. A second layer of adhesive is disposed on the second faceof the substrate layer. A first layer of paper is disposed over thefirst adhesive layer and fixed permanently to the substrate layer and tothe layer of color indicia by the first adhesive layer, while a secondlayer of paper is disposed over the second adhesive layer andpermanently fixed to the substrate layer by the second adhesive layer.

In accordance with a further aspect of the invention, there is provideda security document that includes a resinous substrate sheet having twofaces. Indicia are disposed on one of the faces of the substrate sheet.The security document also includes two paper sheets, each having aninner face and an outer face. One of the paper sheets is permanentlylaminated on each of the faces of the substrate sheet by means of anadhesive applied between the faces of the substrate sheet and the innerfaces of the paper sheets. A set of indicia is printed on at least oneof the outer faces of the paper sheets, whereby the indicia on thesubstrate sheet cooperate with the set of indicia printed on the outerface of the paper sheet to form a total image when viewed in transmittedlight.

In accordance with still another aspect of the present invention, thereis provided a method for manufacturing a security paper including afirst step of defining a graphic design in color. The graphic design isthen printed on a resinous substrate sheet as a substantiallytransparent color image. The resinous substrate sheet is then laminatedbetween two thin paper sheets by means of a suitable adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thefollowing detailed description, taken in conjunction with theaccompanying drawings, wherein like reference numerals refer to likeparts, in which:

FIG. 1 is a partial vertical section of a laminated security paper inaccordance with the present invention, depicting the various layers ofthe laminate and their interaction in transmitted light;

FIG. 2 is a partial vertical section of a laminated security paper inaccordance with the invention, illustrating the reflection, absorption,and transmission of light incident on one surface of the paper;

FIG. 3 is a perspective view of a banknote incorporating the securitypaper in accordance with the invention, with portions of the paperremoved to illustrate certain cooperative effects possible betweenindicia on the various sheets or layers of the paper; and

FIG. 4 is a perspective view of a banknote incorporating the securitypaper of the invention, with portions of the paper removed to show afurther cooperative effect possible between the indicia.

FIG. 5 is a partial vertical section of a laminated security paper inaccordance with one embodiment of the invention, illustrating a layer ofwhite, or substantially opaque ink, above and below the transparentcolor indicia.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings and referring to FIG. 1, a laminatedsecurity paper, designated generally by the reference numeral 10, isdepicted in partial vertical section to illustrate the layers of whichthe paper 10 is comprised. As depicted in FIG. 1, the security paper 10is intended ultimately to be incorporated in a final document 11, suchas a banknote, stock certificate, check, passport, or any otherintrinsically valuable document for which authenticity ensuring featuresare desirable. It should be noted that the present security paper 10 mayalso find applications in other areas as well, including corporateletterhead, paper for occasional invitations, and the like. The securitypaper 10 includes a first paper sheet 12, a substrate sheet 14, and asecond paper sheet 16. Indicia, designated collectively by the referencenumeral 18, are disposed on one surface or face of the substrate sheet14 prior to lamination of the three sheets 12, 14, 16. A set of indicia20, 22 may be disposed on the outer faces 24, 26 of either, or both ofthe two paper sheets 12, 16, as illustrated in FIG. 1. Althoughreference is made to the indicia 18, 20, 22 in the plural throughout thepresent description and in the claims, it should be understood that the"indicia" 18, 20, 22 may, in fact, constitute a single "indicium," orimage, on the substrate sheet 14 or on either of the paper sheets 12,16. As will be more fully described below, the indicia 18 on thesubstrate sheet 14 are preferably designed and disposed on the substratesheet 14 in such a way as to register and cooperate with some, or all ofthe set or sets of indicia 20, 22 on the paper sheets 12, 16 to form a"total image," such as a color picture, when viewed in transmittedlight.

The substrate sheet 14 is preferably a thin, transparent, resinous sheetmaterial. Synthetic materials, such as polyester or polypropylene, ororganic resinous materials such as cellophane, may be used for thesubstrate sheet 14. At present, polyester film material is generallypreferred for the substrate sheet 14, such as a polyester filmmanufactured by E. I. Du Pont de Nemours Company and commerciallyavailable under the tradename Mylar. The substrate sheet 14 preferablyhas a basis weight of between 11.0 and 22.5 grams per square meter,inclusive, and a thickness, or caliper of from 8 to 16 microns. For thebest performance, both during later converting operations and during usewhen incorporated into the laminated security paper 10, it is desirablethat the substrate sheet have good dimensional stability, of the orderof -1.5% (150° C., 30 min.). Information on such dimensional stabilityproperties are commonly available from film suppliers. Resinous sheetmaterial meeting these specifications is available commercially in anumber of forms, including rolls, from a variety of manufacturers,including E. I. Du Pont de Nemours and Company, Imperial ChemicalIndustries Plc, Eastman Kodak Company, and 3M.

The paper sheets 12, 16 incorporated in the security paper 10 arepreferably high opacity, thin paper. The basis weight, or grammage, ofthe paper sheets 12, 16 is preferably in a range from 19 to 50 grams persquare meter, inclusive, with a grammage of about 34 grams per squaremeter being presently preferred. The thickness of the sheets 12, 16 ispreferably included in a range from 0.038 millimeters to 0.050millimeters, inclusive. The paper sheets 12, 16 may typically includefibrous raw materials, such as cotton, flax, hemp, and wood fibers, or amixture of such fibers and cellulosic materials, such as wood cellulose.The paper composition presently preferred is a combination of 75% cottontextile fibers and 25% wood cellulose. However, it should be understoodthat the present security paper 10 is not limited to any particularcomposition of the paper sheets 12, 16, and in specific applications,the precise composition of the paper sheets 12, 16 may be specified bythe eventual issuer of the final document 11. In such cases, additionalsecurity features (not shown), such as polyester, rayon, or nylonfibers, fine huing pigment, and certain chemical additives may beincorporated in the paper sheets 12, 16 without substantially affectingthe operation of the present color marks as security features.

The opacity of the paper sheets 12, 16 is preferably at least 75%, withopacities in a range of from 75 to 85% providing good results whenincorporated in the laminated security paper 10. To obtain such opacitylevels in very thin paper, fillers and pigments may be advantageouslyadded during the papermaking operation. Titanium dioxide may be suitablyemployed as a pigment, along with numerous other substances, such ascalcium carbonate. One such titanium dioxide filler is commerciallyavailable under the tradename "Titanox" RA 50 from Kronos of Canada.

The paper sheets 12, 16 may be manufactured by any suitable papermakingprocess. While Fourdrinier machines are generally preferred for themanufacture of the paper sheets 12, 16, due to the high quality papersuch machines are capable of producing, other types of machines, such ascylinder machines may also be used. For good wet strength properties,wet strength resins are preferably added during the papermaking process.Such resins include melamine-formaldehyde resins, such as resinsavailable from American Cyanamid Company and marketed under thetradename "Parez" resins 607 and 613, or resins available from HerculesInc. under the tradename Kymene, such as Kymeme 450.

Once the substrate indicia 18 have been disposed on the substrate sheet14, as will be described in greater detail below, the paper sheets 12,16 are laminated on either face of the substrate sheet 14. To laminatethe paper sheets 12, 16 to the substrate sheet 14, a suitable adhesiveis applied between the faces of the substrate sheet 14 and the innersurface of the paper sheets 12, 16, and the three sheets 12, 14, 16 arepressed in a laminating machine where the adhesive is cured, as will bedescribed below.

Good adhesion of the paper sheets 12, 16 to the substrate sheet 14 isimportant to the effectiveness of the security paper 10 in preventing ordeterring counterfeiting attempts. Specifically, the laminated securitypaper 10 should not be susceptible of delamination that could expose thesubstrate indicia 18 to copying. Any suitable adhesive that, when curedin the laminate, will preclude delamination of the security paper 10 maybe used. The adhesive may be a single component adhesive, or a double ormultiple component glue. Ultraviolet curing or electron beam curingadhesives may also be employed. Because the substrate indicia 18 shouldselectively cooperate with the surface indicia 20, 22, withoutinterference from the adhesive, the adhesive is preferably transparent.

The particular adhesive employed should be compatible with the substratematerial and with the paper sheets used in the security paper 10.Adhesive manufacturers can generally supply compatibility andsuitability information of this type. Depending upon the specificadhesive used, adhesive may be applied either to paper sheets 12, 16, orto the faces of the substrate sheet 14, or to both the paper sheets 12,16 and the substrate sheet 14. Polyurethane based adhesives aregenerally preferred, and an adhesive available from Imperial ChemicalIndustries Plc of Great Britain, and marketed under the tradenameNovacote ADH 222 has been employed satisfactorily. The coverage rate ofadhesive is preferably within the range of 1.5 to 3.0 grams per squaremeter, but consumption rates may vary with the type of adhesive used.

The substrate indicia 18 are preferably disposed on the substrate sheet14 by printing. For this, a graphic design for the indicia 18 isprepared according to any motif desired in the final security document11. These indicia 18 will often comprise an image, as illustrated inFIGS. 3 and 4, but may also include numerals, words, symbols, or anynumber of these in combination. The indicia 18 may comprise black andwhite indicia, or may include a full range of visible colors, and maycover the entire surface of the substrate sheet 14, or may extend overonly certain regions. Where the indicia 18 on the substrate sheet 14 areintended to cooperate with a set of indicia 20, 22 on either of thepaper sheets 12, 16, transparent ink is generally preferred to definethe substrate indicia 18. As will be discussed in more detail below, theuse of transparent ink permits an observer viewing the security paper10, or the final document 11, in transmitted light, to perceive a totalimage in color, created by cooperation of the transparent color indicia18 on the substrate sheet 14 and the set of indicia 20, 22 on the papersheet 12, 16.

The substrate indicia 18 may advantageously include a layer of white, orsubstantially opaque ink above and/or below the transparent colorindicia. Such white layers are preferably disposed on both sides of thetransparent color ink. Security paper 10, as shown in FIG. 5, includes alayer of white, or substantially opaque ink 61 disposed above and belowthe transparent color indicia 18. These white layers serve twofunctions. First, they increase the opacity of the security paper 10,while nevertheless permitting the substrate indicia 18 to be viewed intransmitted light. Secondly, such layers provide an additional securityfeature in case the security paper 10 is delaminated. While the securitypaper 10 is extremely resistant to any efforts at delamination, should apaper sheet 12, 16 be removed by counterfeiters attempting to copy thesubstrate indicia 18, the white layers would preclude reproduction ofthe color indicia. Any further effort to access the color indicia byremoval of the white layer would result in destruction of the colorindicia, thwarting the counterfeiting enterprise.

An important aspect of the present security paper 10 is its ability topermit verification of authenticity by any holder and under normal lightconditions. Because the human visible spectral range corresponds tolight wavelengths from about 380 to 720 nanometers, the inks employedfor the substrate indica 18 are preferably colored, transparent inks,whose colors also correspond to wavelengths within this range. Use oftransparent inks for substrate indicia 18 is also advantageous becausepractically no light will be reflected from indicia 18, thereby makingphotoreproduction of indicia 18 of the quality required forcounterfeiting impossible.

Other optical effects may be obtained in the security paper 10 byprinting some or all of the indicia 18 with fluorescent orphosphorescent inks, such as inks visible under ultraviolet light only.Where ultraviolet inks are used to form part of the substrate indicia18, indicia printed with such ink may be substantially undetectableunder normal conditions, but visible under ultraviolet light.Alternatively, selected portions of the substrate sheet 14 may be madeopaque by the use of non-transparent inks, such as non-transparent whiteink, or by metallizing, for instance by vapor metal deposition on thesurface of the substrate sheet 14. The use of the latter techniques isparticularly useful in forming microtext as part of the indicia 18,clearly visible in the security paper 10 only in strong transmittedlight, but substantially undetectable in reflected light. Finally, theindicia 18 may advantageously include areas primed with magnetic inkthat serves the dual purpose of forming a part of the total imageobserved in transmitted light and providing an additional securityfeature detectible by magnetically sensitive detection devices of thetype generally known in the art.

The design of the substrate indicia 18 in the security paper 10 permitsa great degree of flexibility in the design of the security feature thatthe final document 11 will incorporate by virtue of the total imagementioned above. For example, the indicia 18 may present areas thatresemble watermarks in the final document, but in any visible color orany combination of colors. Alternatively, color indicia 18 may add colordetail to a set of indicia 20, 22 provided on one or both outer faces24, 26 of the paper sheets 12, 16 when viewed in transmitted light. Insuch cases, at least portions of the surface indicia 20, 22 may also beprinted with transparent inks. As will be discussed in greater detailbelow, where transparent inks are used in both the substrate indicia 18and surface indicia 20, 22, the color of the substrate indicia 18 willdominate in the total image when viewed in strong transmitted light,whereas the color of the surface indicia 20, 22 alone will be visiblewhen the final document 11 is viewed in reflected light. This is trueeven where substantially opaque layers are disposed adjacent the colorregions, as has been described. Portions of the surface indicia 20, 22may become substantially invisible when viewed in transmitted light, dueto the dominance of the substrate indicia 18. Similarly, substrateindicia 18 in transparent colors may provide a colored background, suchas a rainbow or color corporate logo, against which surface printedindicia 20, 22, appear when viewed in transmitted light. For instance,surface indicia 20, 22 may be line art or printed words, such as acompany name, not necessarily overlying substrate indicia 18. As willbecome apparent through the following discussion, although disposed ononly one face of the substrate, 14, the substrate indicia 18 mayregister and cooperate with surface indicia 20, 22 on either of thepaper sheets 12, 16, or on both paper sheets.

FIGS. 1 and 2 illustrate in greater detail the operation of the variouslayers of the security paper 10 under reflected and transmitted lightregimes. As shown in FIGS. 1 and 2, the laminated security paper 10includes a substrate layer 14, a layer of indicia 18 disposed on one ofthe faces of the substrate layer 14, a first paper layer 12 permanentlyfixed over the layer of indicia 18, and a second paper layer 16permanently fixed over the other face of the substrate layer 14.Adhesive layers 28, 30 serve to bind the substrate layer 14 to the paperlayers 12, 16. As described above, in the final document 11incorporating the security paper 10, surface indicia 20, 22 are printedon the exposed surface 24, 26 of either or both of the paper layers 12,16.

Light, such as white light including components of wavelengths betweenabout 380 and 720 nanometers, incident on the security paper 10 iseither transmitted through the paper, absorbed by the paper, orreflected. As represented by the line 32 in FIG. 2, transmitted lightpasses through all layers of the security paper 10 and emerges from thesurface 24 of the paper opposite the surface 26 through which the lightentered. Absorbed light, represented by the line 34 in FIG. 2,penetrates into the security paper 10, but is dispersed and absorbed bythe paper fibers, and by any pigments and fillers included in the paperto increase opacity. Reflected light, represented in FIG. 2 by the line36, does not penetrate into the security paper 10, but rebounds from thesurface 26 of the paper and from any indicia 22 applied to the surface26.

In the security paper 10 as illustrated in FIG. 2, an observer situatedon the same side of the paper as a light source (not shown) from whichthe lines 32, 34, 36 might originate, perceives only the reflected light36, whereas an observer on the opposite side of the security paper 10from the light source sees transmitted light 32. Because absorbed light34 is effectively trapped in the layers of the security paper 10, it isnot perceived by either observer. Due to the high opacity of the paperlayers 12, 16 in the present security paper 10, virtually all lightpenetrating into the security paper 10 is either transmitted through thelayers of the security paper 10, or absorbed in the paper layers 12, 16.Because practically no light reflects from the substrate indicia 18, theindicia 18 remain virtually imperceptible to an observer examining thesecurity paper 10 in reflected light, and photoreproduction of thesubstrate indicia 18 of the quality required for counterfeiting isimpossible.

The cooperation among the substrate indicia 18 and the surface indicia20, 22 in transmitted light is illustrated in FIG. 1 and may besummarized as follows. Some amount of the light 38 incident on thesecurity paper 10 penetrates into the paper through the surface 26 andis transmitted through the paper layer 16, the adhesive layer 30, andthe substrate layer 14 to impact the substrate indicia 18. Regions 40,42, 44, 46, 48 of transparent color in the substrate indicia 18, whosecolor preferably corresponds to the visible spectral colors havingwavelengths of between about 380 and 720 nanometers, absorb certainfrequencies of the transmitted light depending upon the color oftransparent ink applied to the region, and transmit the remainder of thelight. This colored light, represented in FIG. 1 by the shaded areas, isthen transmitted through the adhesive layer 28 and the paper layer 12,and emerges from the surface 24 of the security paper 10.

Where the surface indicia 20 comprise no transparent, color indicia, thelight emerging from the security paper 10 is of substantially the samecolor as the region 40, 42, 44, 46, 48 of the substrate indicia 18through which the light passed. In areas of the surface indicia 20covered by transparent colored ink, the color of the transmitted lightemerging from the paper is primarily that of the substrate regionsunderlying those areas. The total image observed emerging from thesecurity paper 10 in transmitted light is, therefore, a combination ofthe image created by the substrate indicia 18 and the surface indicia20. The stronger the transmitted light 38 incident on the security paper10, the more the image created by the substrate indicia 18 will tend todominate over the image created by the surface indicia 20.

FIG. 1 also illustrates how other optical effects may be incorporatedinto the security paper 10. In lieu of transparent ink, the substrateindicia 18 may comprise regions 54 made opaque by metallizing or byprinting with non-transparent ink. Such regions 54 will, of course,effectively stop all transmitted light and thus produce dark regions inthe total image perceived. Alternatively, the substrate 14 may beperforated, in certain regions 56. Such perforations will become filledwith adhesive during the lamination of the various layers of thesecurity paper 10. Because the adhesive is preferably transparent, suchperforated regions 56, will transmit light without alteration.

Examples of these cooperative effects are illustrated in FIGS. 3 and 4.In FIG. 3, line art comprising the numeral "1" surrounded by a circle(depicted by dashed lines 60) is shown as it might be printed in a lowercorner of the outer surface 26 of the paper sheet 16, with a similarencircled numeral "1" being printed in an upper corner of the samesurface 26. Colored regions 62 (only one such region shown in FIG. 3) onthe substrate sheet 14 are positioned to register with and to overliethe encircled numeral on the paper surface 26. In this arrangement, anobserver viewing the outer surface 26 of the final document 11 inreflected light would perceive only the surface indicia 22, 60. When thesame document 11 is viewed from the same side, but with a light sourcelocated on the opposite side of the document from the observer, thetotal image perceived by the observer would include the line art 60,colored by the colored regions 62 on the substrate.

A similar arrangement is depicted in FIG. 3 for substrate indicia 18representing a building and landscape 64. An image 66 printed on theouter surface 24 of the paper sheet 12 may include line art andtransparent ink covering some or all of the surface of the image 64. Asin the previous example, an observer viewing the document 11 inreflected light would perceive only the image 66 as it appears on thesurface 24 of the paper sheet 12, and in the colors printed on thesurface 24. Viewing the document 11 in transmitted light, however, theobserver would perceive a total image made up of the surface image 66combined with the substrate image 64. Generally, the greater theintensity of the light used to view the document 11 in the transmittedlight regime, the more the substrate image 64 will dominate in the totalimage perceived by the observer. Where desired, the surface indicia mayeven become substantially invisible when the document is viewed intransmitted light, due to the dominance of the substrate image 64 in thetotal image.

Another example of the cooperative effects of the substrate indicia 18and the surface indicia 20, 22 is illustrated in FIG. 4. As mentionedabove, the design and placement of the substrate indicia 18 need not beidentical to the design and placement of the surface indicia 20, 22. Asshown in FIG. 4, a security feature incorporated in the security paper10 may consist in a "ghosting" effect induced by the substrate indicia68 when viewed in transmitted light. In such cases, the surface indicia20, 22 need not include line art or other indicia specifically "colored"by the substrate indicia 18 in the total image.

The preferred method for making the security paper 10 will now bedescribed. Because the substrate indicia 18 form a total image incombination with the surface indicia 20, 22 disposed on the securitypaper 10 in the final document 11, the process for manufacturing thesecurity paper 10 generally begins with the graphic design of allindicia 18, 20, 22 to be printed on the substrate sheet 14 and papersheets 12, 16. Alternatively, the graphic design for the substrateindicia 18 may be prepared independently of the surface, indicia 20, 22,particularly where a standard paper is used by an issuer for a number ofdifferent final documents. In either case, the graphic design for thesubstrate indicia 18 may be prepared by an artist-designer based on anycolor picture or motif, and may incorporate a full range of visiblecolors and details. In developing the graphic design, the designer mayspecify that the design be precisely placed in the security paper 10, orcontinuously or repetitively printed on the substrate 14. As has beendescribed above, the graphic design for the substrate indicia 18 mayinclude areas of color over which surface indicia 20, 22, includingtext, will be printed in the final document 11.

In addition to color marks and images, the graphic designer may includein the substrate indicia 18 regions of microtext, portraits or wordsresembling water marks printed in non-transparent or transparent whiteink, and designations printed in fluorescent, phosphorescent, ormagnetic ink. Moreover, the design may include perforations in thesubstrate sheet 14 designed to permit transmitted light to passunaltered. Metallized regions may also be provided in the design torender the substrate sheet 14 opaque.

The graphic design is processed in order to convert it to a printableform. This is preferably accomplished by electronic color separation. Anumber of suitable color separation devices are known, including laseranalyzers and scanners. Such devices transform the design into pixels,or an array of minute points, containing primary colors. The colorseparation devices also determine the intensity of the coloration of thepixels, and encode and store the image thus analyzed.

Once the color separation is complete, priming plates or cylinders areprepared for printing the substrate indicia 18 onto one face of thesubstrate sheet 14. The preferred priming process is rotogravurereproduction. The electronic color separation device is preferably usedeither for direct cylinder engraving, or for controlling the exposuretime of photosensitive material in an indirect process for chemicallyengraving a rotogravure cylinder. Color synthesis in the printedsubstrate indicia 18 is accomplished by means of separate rotogravurecylinders for each transparent color, and for any non-transparent ormagnetic inks called for in the graphic design. The engraving depth onthe rotogravure, cylinder is typically in a range of from 3 to 32microns.

Prior to printing the substrate indicia 18 on the substrate sheet 14,the substrate sheet is preferably given a Corona electric dischargetreatment to improve the adherence of the ink to the substrate sheet 14.Corona treatment has also been found to enhance the adherence of thepaper sheets 12, 16 that will be laminated on the substrate sheet 14later in the manufacturing process. This Corona treatment may be carriedout in any suitable apparatus readily available commercially fortreating thin films, such as treaters available from Sherman TreatersNorth America Inc.

Optionally, the substrate sheet 14 may be metallized or perforated priorto printing, if called for in the graphic design of the substrateindicia 18. Vapor deposition techniques are preferred for metallizing,and all or a portion of the substrate sheet 14 may be exposed to suchdeposition. Perforation of the substrate sheet 14 may be used to formtext, specific designs, or dotted contours to provide an added securityfeature. Such perforation is generally carried out on specialperforating machines having needles, typically 0.8 to 1.0 millimeters indiameter, that pierce the substrate sheet 14. Where a perforated patternis designed to cooperate with a printed or metallized area on thesubstrate sheet 14, the perforating step is preferably carried out aftercompletion of the printing and metallizing; steps. Such perforation maybe carried out on any suitable machine, such as machines availablecommercially from Zimmermann & Co. of Berlin, Germany.

The substrate sheet 14 is then printed with the substrate indicia 18,allowing sufficient time between printing stages to allow inks to dry.For printing, as for the other conversion steps in the preparation ofthe security paper 10, the substrate sheets 14 preferably processed froma roll of resinous film or foil. Processing the substrate material inroll form facilitates the printing and lamination steps, and permits theprocess to be carried out in production quantities and at a reasonablecost.

The printed substrate sheet 14 is next laminated between two papersheets 12, 16 to form the security paper 10. In a presently preferredarrangement for laminating the security paper 10, lamination isaccomplished in a laminating station consisting of a laminating machineof the type generally known, and a tunnel-type adhesive curing section.Such laminating machines are commercially available from a number ofmanufacturers, such as Cerruti of Italy. To form the laminate, twosheets of thin, high opacity paper are fed into the laminating station,preferably from continuous rolls, and the printed substrate sheet 14 isfed into the station between the paper sheets. Adhesive is continuouslyapplied to either the faces of the substrate sheet, or to the surfacesof the paper sheets facing the substrate sheet depending upon theadhesive employed, and the sheets are pressed by nip rolls to form alaminate. The laminate proceeds through the curing section where theadhesive is cured to permanently bond the laminated layers. The precisedesign of the curing section depends upon the adhesive employed, and thecuring conditions recommended by the adhesive manufacturer.

The security paper, exiting from the laminating station as a continuoussheet, may be rolled for later cutting into sheets, or may be sheeteddirectly as it is produced. Any suitable sheeting device may be employedfor this, such as a Pasaban sheeter. The format set on the sheeter willdepend upon specifications set by the end user of the security paper 10,and ultimately upon the size of the document to be made from thesecurity paper.

After sheeting, counting and quality control operations may beperformed, and the surface indicia 20, 22 may be printed on one or bothsides of the security paper 10. The security paper 10 is suitable forintaglio printing, generally preferred for such documents as banknotes,as well as for multicolor offset printing. Where registration betweenthe substrate indicia 18 and the surface indicia 20, 22 is desired,registration marks are preferably incorporated in the substrate indicia18, and printing on the security paper 10 is registered in generally thesame manner as for known water marks.

The foregoing method has been found to produce high quality securitypaper with physical and optical properties generally comparable to orsurpassing those of known security papers. The substrate indicia 18 aresubstantially undetectable when the security paper 10, or the finaldocument 11, is viewed in reflected light, and clearly visible whenviewed in transmitted light. The stability of the colors comprising thesubstrate indicia 18 is very good, particularly where similar pigmentsare used to print these indicia 18 as are employed for making banknotesand other security documents. Because the indicia 18 are printed on thesubstrate sheet 14, and the substrate indicia 18 are separated from thepaper sheets 12, 16 by the adhesive layers 28, 30 in the laminatedsecurity paper 10, show-through of the substrate indicia 18 is avoided.Moreover, the rate at which the present security paper can be producedexceeds the production rate typical for security papers bearing watermarks.

The security paper 10 offers a number of additional security features ascompared with known security papers. Due to the practical difficulty inprinting on thin resinous films, and in producing very thin, highopacity papers, only a tiny sector of the public will possess theknowledge and machinery needed to replicate the security paper. Inaddition, the inability to photoreproduce the substrate indicia 18effectively precludes duplication by photocopying. On the other hand,the total image perceived by observing the final document 11, producedfrom the security paper 10, in transmitted light within the visiblespectrum, provides a simple and effective means by which any holder mayverify the authenticity of the document under ordinary light conditions.

Security paper in accordance with the foregoing description was preparedand tested. Results of those tests are summarized in the following fourexamples and in Table 1, along with results of similar tests oncommercially available security papers. Because certain of the tests mayhave been performed in accordance with non-standard test methodologies,these results are provided for comparative purposes only. Moreover, aswill be recognized by those skilled in the art, the results of suchtests may not always be reproducible as between testing machines andlaboratories.

Tests on the physical and optical properties of the papers was performedin an atmosphere of 50 to 51% humidity and at a temperature of between22.5° and 23° C.

Smoothness was tested in accordance with the Bekk method on equipmentmanufactured by Karl Frank GmbH of Germany, and is reported in seconds(sec). The Bekk method is summarized in test specification number T 479om-86 of the Technical Association of Pulp and Paper Industry (TAPPI).

Rigidity was tested on a Buchel stiffness tester, and results arereported in millinewtons (mN) of force.

Breaking strength was tested in accordance with the Mullen test methodon equipment manufactured by Karl Frank GmbH. The Mullen test method issummarized in test specification number T 403 om-85 of the TAPPI.Results are reported in kilopascals (kPa).

Tearing strength was tested in accordance with the Elmendorf method onequipment manufactured by Lorentzen-Wettre of Sweden on paper samplesmeasuring 65×80 millimeters. The Elmendorf method is summarized in testspecification number T 414 om-88 of the TAPPI. Results are reported indecanewtons (daN).

Opacity was tested on an Elrepho Datacolor 200 test machine, and resultsare reported in percent (%).

Finally, the average number of double folds was determined usingequipment manufactured by Karl Frank GmbH, with a weight of 1 kilopond.Results are reported in the number of double folds before breaking.

EXAMPLE 1

A portrait of a historical figure was used for the motif of a color markin the form of a line drawing. A polyester substrate sheet, 12 micronsthick, available under the tradename Yuborlen, was given a coronatreatment on both sides, and the color mark and a color test spectrumwere printed in four basic colors by rotogravure printing. Paper sheets,made of cotton fibers and having a basis weight of 32 g/m², werelaminated successively on each side of the substrate sheet usingpolyurethane based adhesive commercially available under the tradenameNovacote ADH 222 from ICI of Great Britain.

EXAMPLE 2

A color icon was color separated and printed in four colors on the sametype of polyester substrate as in example 1. Paper made of hemp fiber,with a basis weight of 30 g/m², was laminated to both sides of thesubstrate sheet as in the previous example.

EXAMPLE 3

The portrait of the historical figure and the icon of the previous twoexamples were printed by five color rotogravure printing on the sametype of substrate used in the previous examples. Paper comprising cottonfibers and wood cellulose, and having a basis weight of 35 g/m², werelaminated to both sides of the substrate sheet using the polyurethaneadhesive of the previous examples.

EXAMPLE 4

For this example, a polyester film, 12 microns thick and with a basisweight of 16.9 g/m², available under the tradename Mylar, was used. Amulticolor, semihue ornament with a single color text in red, blue andblack, and a single color continuous ornament of white, opaque ink,similar to a water mark, were printed the film by rotogravure printing.The printed substrate film was laminated with paper sheets having abasis weight of 36 g/m² and consisting of 75% cotton fibers and 25% woodcellulose, using Novacote ADH 222 adhesive as before.

As the figures in Table 1 indicate, the security paper produced in theexamples exhibits physical and optical properties comparable to, orexceeding commercially available security papers. In particular, thepresent security paper has excellent strength and durabilitycharacteristics, including a greatly enhanced folding endurance. Theindicia printed on the substrate is substantially undetectable underreflected light and no show-through is apparent. When viewed intransmitted light within the visible spectrum, the substrate indicia areclearly visible.

                                      TABLE 1                                     __________________________________________________________________________              PORTALS ARJOMARI                                                                             LOUISENTHAL                                                    [Great Britain]                                                                       [France]                                                                             [Germany]                                                                              EXAMPLE 1                                                                            EXAMPLE 2                                                                            EXAMPLE                                                                              EXAMPLE                __________________________________________________________________________                                                           4                      BASIS WEIGHT                                                                            83      80     82       84     85     88     93                     [gr/m.sup.2 ]                                                                 THICKNESS 0.108   0.100  0.098    0.095  0.096  0.104  0.111                  [mm]                                                                          SMOOTHNESS                                                                              9.3     12.6   10.5     10.8   11.3   11.7   12.03                  [sec]                                                                         RIGIDITY  93      100    98.7     70     71     76     74                     [mN; 10 mm,                                                                   15°]                                                                   BREAKING  463     425    432      415    420    435    495                    STRENGTH                                                                      [kPa]                                                                         TEARING   12.16   11.87  11.90    12.40  12.47  12.75  13.43                  STRENGTH                                                                      [daN]                                                                         OPACITY [%]                                                                             88.19   89.50  85.31    89.85  91.70  88.60  93.23                  AVERAGE NO.                                                                             1097    1570   1465     over 2000                                                                            over 2000                                                                            over 2000                                                                            over 2000              OF DOUBLE                                                                     FOLDS                                                                         __________________________________________________________________________

We claim:
 1. A security paper comprising:a resinous substrate sheethaving two faces; transparent indicia disposed on one of said faces ofsaid resinous substrate sheet; and, a first and a second paper sheet,said first paper sheet being permanently laminated on one of said facesof said resinous substrate sheet, and said second paper sheet beingpermanently laminated on the other of said faces of said resinoussubstrate sheet, whereby said indicia are substantially undetectablewhen viewed in reflected light and are apparent through said papersheets when viewed in transmitted light.
 2. The security paper as setforth in claim 1, wherein said paper sheets are permanently laminated oneach of said faces of said substrate sheet by means of an adhesive. 3.The security paper as set forth in claim 1, wherein said indicia includeat least one color region, the color of said region corresponding to awavelength of light of from 380 to 720 nanometers.
 4. The security paperas set forth in claim 3 further comprising a first layer ofsubstantially opaque ink disposed under said indicia and a second layerof substantially opaque ink disposed over said indicia.
 5. The securitypaper as set forth in claim 1, wherein a portion of said resinoussubstrate sheet is metallized.
 6. The security paper as set forth inclaim 1, wherein a portion of said resinous substrate sheet isperforated.
 7. The security paper as set forth in claim 1, furthercomprising microtext disposed on said resinous substrate sheet.
 8. Thesecurity paper as set forth in claim 1, wherein said indicia are printedon said resinous substrate sheet with transparent ink.
 9. The securitypaper as set forth in claim 8, wherein at least a portion of saidindicia is printed in color.
 10. The security paper as set forth inclaim 1, wherein at least a portion of said indicia is printed on saidresinous substrate sheet with fluorescent ink.
 11. The security paper asset forth in claim 1, wherein at least a portion of said indicia isprinted on said resinous substrate sheet in ultraviolet ink, saidportion of said indicia being substantially undetectable under lightwithin the visible spectrum and visible when viewed under ultravioletlight.
 12. The security paper as set forth in claim 1, wherein at leasta portion of said indicia is printed on said resinous substrate sheetwith phosphorescent ink.
 13. The security paper as set forth in claim 1,wherein said resinous substrate sheet comprises a resinous materialselected from the group consisting of polyester, polypropylene andcellophane.
 14. The security paper as set forth in claim 1, wherein saidresinous substrate sheet has a basis weight included in a range from11.0 to 22.5 grams per square meter inclusive.
 15. The security paper asset forth in claim 1, wherein each of said paper sheets has a basisweight included in a range from 19 to 50 grams per square meterinclusive, a thickness included in a range from 0.038 to 0.050millimeters inclusive, and an opacity included in a range from 75 to 85percent inclusive.
 16. The security paper as set forth in claim 1,wherein each of said paper sheets comprises fibrous material selectedfrom the group consisting of wood fibers, cotton fibers, flax fibers,and hemp fibers.
 17. A laminated security paper comprising:a resinoussubstrate layer having a first face and a second face; color indiciaprinted on said first face of said substrate layer with transparent ink,said color indicia including at least one region having a colorcorresponding to a wavelength of light of from 380 to 720 nonometers; afirst layer of adhesive disposed on said first face of said substratelayer, over said color indicia; a second layer of adhesive disposed onsaid second face of said substrate layer; a first layer of paperdisposed over said first adhesive layer and fixed permanently to saidsubstrate layer and said color indicia by said first adhesive layer; anda second layer of paper disposed over said second adhesive layer andfixed permanently to said substrate layer by said second adhesive layer.18. The laminated security paper as set forth in claim 17 furthercomprising a first white layer disposed under said color indicia, and asecond white layer disposed over said color indicia.
 19. The laminatedsecurity paper as set forth in claim 17, wherein said resinous substratelayer has a basis weight included in a range from 11.0 to 22.5 grams persquare meter inclusive, and wherein each of said paper layers has abasis weight included in a range from 19 to 50 grams per square meterinclusive, a thickness included in a range from 0.038 to 0.050millimeters inclusive, and an opacity included in a range from 75 to 85percent inclusive.
 20. The laminated security paper as set forth inclaim 17, wherein at least a portion of said color indicia is printed onsaid resinous substrate layer with fluorescent ink.
 21. The laminatedsecurity paper as set forth in claim 17, wherein at least a portion ofsaid color indicia is printed on said resinous substrate layer withphosphorescent ink.
 22. The security document comprising:a resinoussubstrate sheet having two faces; transparent indicia disposed on one ofsaid faces of said resinous substrate sheet; a first and a second papersheets, each of said paper sheets having an inner face and an outerface, one of said paper sheets being permanently laminated on arespective face of said resinous substrate sheet by means of an adhesiveapplied between said respective face of said substrate sheet and saidinner face of each of said paper sheets; and a set of indicia printed onat least one of said outer faces of said paper sheets, whereby saidtransparent indicia on said substrate sheet cooperate with said set ofindicia to form a total image when viewed in transmitted light.
 23. Thesecurity document as set forth in claim 22, wherein at least a portionof said indicia printed on said outer face of said paper sheet becomesubstantially invisible when viewed in transmitted light.
 24. Thesecurity document as set forth in claim 22, wherein said set of indiciadisposed on said outer face of said paper sheet is in register with saidindicia on said substrate sheet.
 25. The security document as set forthin claim 22 further comprising a first layer of substantially opaque inkdisposed under said transparent indicia and a second layer ofsubstantially opaque ink disposed over said transparent indicia.
 26. Thesecurity document as set forth in claim 22, wherein each of said papersheets has an opacity of from 75 to 85 inclusive.
 27. The securitydocument as set forth in claim 22, wherein said resinous substrate sheethas a basis weight included in a range from 11.0 to 22.5 grams persquare meter inclusive, and wherein each of said paper sheets has abasis weight included in a range from 19.0 to 50.0 grams per squaremeter inclusive and a thickness included in a range from 0.038 to 0.050millimeters inclusive.
 28. The security document as set forth in claim22, wherein said transparent indicia include at least one color region,the color of said region corresponding to a wavelength of light of from380 to 720 nanometers.
 29. The security document as set forth in claim22, wherein a portion of said resinous substrate sheet is metallized.30. The security document as set forth in claim 22, wherein a portion ofsaid resinous substrate sheet is perforated.
 31. The security documentas set forth in claim 22, further comprising microtext disposed on saidresinous substrate sheet.
 32. The security document as set forth inclaim 22, wherein said indicia are printed on said resinous substratesheet with transparent ink.
 33. A method for manufacturing a securitypaper comprising the steps of:defining a graphic design in color;printing said graphic design on a resinous substrate sheet as asubstantially transparent color image; and, permanently laminating saidprinted substrate sheet between two paper sheets by means of anadhesive.
 34. The method as set forth in claim 33, comprising thefurther step of metallizing a portion of said resinous substrate sheetprior to said laminating step.
 35. The method as set forth in claim 33,comprising the further step of perforating said resinous substrate sheetprior to said laminating step.
 36. The method as set forth in claim 33,comprising the further step of printing a microtext on said resinoussubstrate sheet prior to said laminating step.
 37. The method as setforth in claim 33, wherein at least a part of said graphic design isprinted on said substrate sheet with fluorescent ink.
 38. The method asset forth in claim 33, wherein said graphic design is printed on saidresinous substrate sheet by a rotogravure printing process.
 39. Themethod as set forth in claim 33 further comprising disposing a firstlayer of substantially opaque ink under said graphic design anddisposing a second layer of substantially opaque ink over said graphicdesign.
 40. The method as set forth in claim 33, wherein at least aportion of said graphic design is printed on said substrate sheet withphosphorescent ink.
 41. A method for manufacturing a security documentcomprising the steps of:defining a graphic design in color; printingsaid graphic design on a resinous substrate sheet as a substantiallytransparent color image; permanently laminating said printed substratesheet between two paper sheets by means of an adhesive; and printing aset of indicia on an outer surface of at least one of said paper sheets,whereby said graphic design on said substrate sheet cooperates with saidset of indicia to form a total image when viewed in transmitted light.42. The method as set forth in claim 41 wherein said set of indicia isprinted on said outer surface of at least one of said paper sheets byintaglio printing.
 43. The method as set forth in claim 41, comprisingthe further step of metallizing a portion of said resinous substratesheet prior to said laminating step.
 44. The method as set forth inclaim 41, comprising the further step of perforating a portion of saidresinous substrate sheet prior to said laminating step.
 45. The methodas set forth in claim 41, comprising the further step of printing amicrotext on said resinous substrate sheet prior to said laminatingstep.
 46. The method as set forth in claim 34, wherein said graphicdesign is printed on said resinous substrate sheet by a rotogravureprinting process.